1. Field of the Invention
The invention relates generally to the correction of exposure variations in an optical system. More particularly, this invention relates to correcting variations introduced by one or more components, such as a reticle or pellicle, into the exposure field of the optical system.
2. Description of the Related Art
Optical systems are widely used in the microelectronics industry to manufacture semiconductor wafers by a process known as photolithography. Typically, a photolithography system comprises a light source configured to project light rays to a condenser lens. The condenser lens collimates the light rays towards a pellicle placed before (or after) a reticle. Typically, the reticle is an optically clear quartz substrate having a chrome pattern used to project an image onto a portion of a photoresist-coated wafer. The pellicle is a very thin, transparent film which seals off the reticle surface from airborne particulates and other forms of contamination. A projection lens is placed after the reticle to receive and focus the light rays onto an exposure field on the wafer.
In designing such an optical system, any one of these components may be vulnerable to manufacturing imperfections which, even if minutely small, may cause intolerable or unacceptable defects in the photoresist layer of the wafer. For example, aberrations due to defects in one or more lenses may include distortion, curvature of field, spherical aberration, and astigmatism. Moreover, distortions may be due to defects in the reticle that may be caused during manufacturing. For example, reticle defects may arise from impurities in the chrome, lack of adhesion of the chrome to the reticle, variances in ion beam used to produce chrome etching, or other similar defects. Reticle defects may cause intolerable or unacceptable variations in critical dimensions (CD's) in the exposure field. A CD represents the width or space of critical elements in an integrated circuit (IC).
Several attempts were made in the industry to compensate for general defects in the optical system. For example, in U.S. Pat. No. 5,640,233 issued to McArthur et al., a two-plate corrector is disclosed in a stepper configuration so that an image from a reticle plane is projected to an ideal image at an object plane. Based on the premise that depth of field correction made at the reticle plane induces insignificant distortion, McArthur describes placement of the two-plate corrector at the reticle plane to correct depth of field distortions caused by the lens system. However, McArthur does not describe how to correct defects resulting from specific components, such as the reticle or pellicle.
To eliminate undesirable variances that result from defects in the reticle, some manufacturers simply replace the defective reticle with a new reticle. Other manufacturers resort to discarding wafers having intolerable CD's caused by the defective reticle. In either case, a significant increase in manufacturing cost due to reticle defects has become unavoidable.
Therefore, there is a need in the industry to compensate for individual component defects without having to replace the component or discard any defective wafers resulting therefrom. The solution of correcting such defects should be cost-effective and easy to implement.